1. Field of the Invention
The present invention relates generally to a Complementary Metal Oxide Semiconductor (CMOS) image sensor and a related method of operation. More particularly, the invention relates to a CMOS image sensor and related method of operation producing reduced afterimage effects.
2. Description of the Related Art
Image sensors find application in a variety of fields, including machine vision, robotics, satellite-based instrumentation, automobiles, navigation and guidance equipment, etc. In general construct, conventional image sensors include a two-dimensional array of pixels formed on a semiconductor substrate. This pixel array defines an image field or image frame.
Each pixel within the plurality of pixels forming the pixel array typically includes a photoelectric conversion element capable of accumulating a quantity of electrical charge in relation to an amount of detected energy (e.g., visible light, etc.). That is, when photons impact the surface of a photoelectric conversion element, free charge carriers are produced. These free charge carriers are subsequently collected by the constituent photoelectric conversion element. Using well understood techniques, the collected free charge carriers are then converted in a read-out operation that allows transfer of a signal (e.g., a voltage or current) corresponding to the quantity of free charge carriers collected. The aggregation of output signals from the plurality of pixels may then be communicated through an output circuit, and subsequently used to generate an image corresponding to the detected energy emanating from the image frame.
Representative conventional image sensors include charge coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensors. As is generally understood, CCDs have lower noise and produce a better quality image than CMOS image sensors. However, CMOS image sensors are more easily operated and better adapted to a variety of scanning techniques. Furthermore, signal processing circuits can be integrated on a single chip with CMOS image sensors, thus enabling miniaturization of the incorporating product. The compatibility of CMOS image sensors with conventional CMOS fabrication processes also reduces manufacturing costs. CMOS image sensors are also characterized by relatively low power consumption. This characteristic makes CMOS image sensors ideal for products having limited battery capacity. As a result of the foregoing advantages, conventional CMOS image sensors have been widely used in such commercial embodiments as display devices having SVGA (0.5 mega pixel) and MEGA (1 mega pixel) resolutions.
Conventional CMOS image sensors may be fabricated with a variety of specific structures, but are generally formed with a structure comprising four transistors and a photodiode. This structure is commonly referred to as a “4Tr structure.” Advantageously, the 4Tr structure may be manufactured using a conventional CMOS fabrication processes.
A conventional CMOS image sensor having a four transistor (4Tr) structure operates in the following manner. The constituent photodiode accumulates electrical charge corresponding to an amount of light energy absorbed. A charge transfer element then transfers the accumulated charge from the photodiode to a charge detection element. An associated amplifier, formed for example by a source follower buffer amplifier and a constant current source, receives an electrical signal from the charge transfer element and outputs a corresponding output signal.
Unfortunately, the transfer of charge from the photodiode in the conventional CMOS image sensor to the charge detection element is often inefficiently or inadequately performed. Residual charge remaining in the photodiode after charge transfer produces so-called “afterimage effects.” This phenomenon has the capacity of producing an erroneous image during a subsequent image read operation. The residual charge also tends to reduce the charge integration (i.e., accumulation) capacity of the photodiode. The conversion gain of the photodiode, (i.e., the amount of charge generated per photoelectron) during a subsequent image read operation is also reduced due the erroneous charge distribution between the photodiode and charge detection element.
As a result of the foregoing, multiple conventional attempts have been made to remediate the problem of lingering afterimage effects. Consider, for example, U.S. Pat. No. 6,140,630. In this patent document, one or more specialized charge pump element(s) are provided in relation to the pixel array element of a CMOS imager. The specialized charge pump elements are used to derive an over-voltage signal from Vdd. Each pixel element within a pixel array, and more particularly the charge transfer element of each pixel element, is connected to a charge pump element in order to receive the over-voltage signal. Unfortunately, this conventional approach suffers from several drawbacks. For example, the specialized and specially provided charge pump elements and associated power signal increase the size and complexity of the pixel array element. The over-voltage signal is also constantly ON when applied to the charge transfer and other elements of the individual pixel units forming the pixel array. Accordingly, the constituent elements must be sized appropriately to deal with the over-voltage signal.
A more efficient (e.g., less brute-force) approach is desired to address the ongoing problem of after-image effects in a CMOS imager. That is, an approach is required that does not adversely impact the overall size of the CMOS imager or hazard its constituent elements with a constantly ON over-voltage signal.